Massive black hole and gas dynamics in galaxy nuclei mergers. I. Numerical implementation

Numerical effects are known to plague adaptive mesh refinement (AMR) codes when treating massive particles, e.g. representing massive black holes (MBHs). In an evolving background, they can experience strong, spurious perturbations and then follow unphysical orbits. We study by means of numerical simulations the dynamical evolution of a pair MBHs in the rapidly and violently evolving gaseous and stellar background that follows a galaxy major merger. We confirm that spurious numerical effects alter the MBH orbits in AMR simulations, and show that numerical issues are ultimately due to a drop in the spatial resolution during the simulation, drastically reducing the accuracy in the gravitational force computation. We therefore propose a new refinement criterion suited for massive particles, able to solve in a fast and precise way for their orbits in highly dynamical backgrounds. The new refinement criterion we designed enforces the region around each massive particle to remain at the maximum resolution allowed, independently upon the local gas density. Such maximally-resolved regions then follow the MBHs along their orbits, and effectively avoids all spurious effects caused by resolution changes. Our suite of high resolution, adaptive mesh-refinement hydrodynamic simulations, including different prescriptions for the sub-grid gas physics, shows that the new refinement implementation has the advantage of not altering the physical evolution of the MBHs, accounting for all the non trivial physical processes taking place in violent dynamical scenarios, such as the final stages of a galaxy major merger.Comment: 11 pages, 11 figures, 1 table, it matches the published versio

Simplified galaxy formation with mesh-less hydrodynamics

Numerical simulations have become a necessary tool to describe the complex interactions among the different processes involved in galaxy formation and evolution, unfeasible via an analytic approach. The last decade has seen a great effort by the scientific community in improving the sub-grid physics modelling and the numerical techniques used to make numerical simulations more predictive. Although the recently publicly available code GIZMO has proven to be successful in reproducing galaxy properties when coupled with the model of the MUFASA simulations and the more sophisticated prescriptions of the Feedback In Realistic Environments (FIRE) set-up, it has not been tested yet using delayed cooling supernova feedback, which still represent a reasonable approach for large cosmological simulations, for which detailed sub-grid models are prohibitive. In order to limit the computational cost and to be able to resolve the disc structure in the galaxies we perform a suite of zoom-in cosmological simulations with rather low resolution centred around a sub-L* galaxy with a halo mass of $3\times 10^{11}\,\rm M_\odot$ at $z=0$, to investigate the ability of this simple model, coupled with the new hydrodynamic method of GIZMO, to reproduce observed galaxy scaling relations (stellar to halo mass, stellar and baryonic Tully-Fisher, stellar mass-metallicity and mass-size). We find that the results are in good agreement with the main scaling relations, except for the total stellar mass, larger than that predicted by the abundance matching technique, and the effective sizes for the most massive galaxies in the sample, which are too small.Comment: 15 pages, 14 figures and 1 table; matches the published versio

Black holes in galactic nuclei: seed formation from stellar mass black holes and massive black hole pairing in galaxy mergers.

Black holes (BHs) are a very important class of astrophysical objects. They are the most compact objects in the Universe, hence they represent the most extreme sources of gravity. BHs come in two flavours: the stellar mass BHs (SBHs) relic of young massive stars (1 1220M 99) and the massive BHs (MBHs), with masses of 106 12109M 99, dwelling in the nuclei of the most massive galaxies. While the formation mechanisms of SBHs are well understood, no clear consensus exists about MBH formation. According to the Soltan arguments (Soltan, 1982), MBHs gain the largest fraction of their mass via radiative efficient accretion of gas. As a consequence, we expect that MBH formed early in the Universe as smaller mass seeds. Recently, observations of high redshift quasars (e.g.; Mortlock et al., 2011; Fan et al., 2006) showed that MBHs with masses above 109M 99 were already in place when the Universe was less than 1 Gyr old and posed tight constraints on the models for the formation and growth of MBHs. Two main scenarios have been developed for MBH seed formation: the light seed scenario, where seeds formed as relic of the first generation of stars with masses of up to few hundred solar masses (Madau & Rees, 2001), and the heavy seed scenarios, where seeds formed from the direct collapse of massive gas clouds in primordial haloes with masses of up to few 105M 99 (Haehnelt & Rees, 1993). Despite the large number of studies about MBH formation models, each model still has its own caveats, which make the study of MBH formation worth of further investigations. According to the \uf04c-CDM cosmology, galaxies form when gas cools down within dark matter haloes, which assembly in a hierarchical fashion from small density perturbations. Galaxies grow via accretion and mergers, and the central MBHs evolve in the same way. So, when a galaxy merger occur, the MBHs hosted in the nucleus of the galaxy progenitors can sink towards the centre of the merger remnant, forming a MBH binary (MBHB). Despite galaxy mergers are usually observed, no clear detections of MBHBs exist to date. The formation and evolution of MBHBs is a complex process, since it occurs in a rapidly varying environment where gas, star formation and SNa feedback play a pivotal role. Several studies have been performed to date, but a clear understanding of the whole process is still far from being reached. In this thesis I cover both aspects of MBH formation and evolution. In the first study I consider an alternative route for seed BH formation. Using two different codes, the AMR code RAMSES (Teyssier, 2002) and the meshfree code GIZMO (Hopkins, 2015), I studied the evolution of a single massive circum-nuclear gaseous disc embedding a population of SBHs. The disc was subject to radiative cooling, star formation and supernova feedback and becomes unstable to fragmentation, which led to the formation of clumps as massive as 104 12 105M 99. My simulations showed that during the disc evolution, some SBHs can be gravitationally captured by a clump. Within the clumps, such BHs can experience episodes of super-critical accretion, which make them grow up to 103 12 104M 99 in few Myr. Thanks to the very low radiative efficiency associated to the slim accretion disc (Abramowicz et al., 1988), the energy released to the surrounding gas is too small to halt the accretion flow, hence BHs can accrete almost unimpeded until one of these events occur: the clump is totally accreted by the BH, the clump is consumed by star formation or the clump is destroyed by supernova explosions. In the second study, instead, I consider the intermediate stages of a galaxy merger, when the MBHs originally dwelling in the centre of their own progenitor galaxies reach few hundred separations in the nucleus of the merger remnant. I assumed that each MBH was embedded in a self-gravitating circumnuclear gaseous disc. With the code RAMSES I studied the evolution of the MBHs and their surrounding discs, including physical processes like radiative cooling, star formation and supernova feedback, which are implemented in the code as sub-grid recipes. First, I implemented a new refinement prescription aimed at improving the orbital evolution of massive particles, an already known major issue in AMR codes, like observed by Gabor & Bournaud (2013); Dubois et al. (2014). Secondly, I evolved the discs assuming different sub-grid recipes to study how the MBH and gas dynamics could be affected by the different choices. I found that the MBH dynamics is almost independent of the physical modelling, if one assumes that no previous star formation occurred in the discs, while the gas evolution and its final distribution can be significantly affected. On the other side, if one assumes that star formation was already ongoing, even the BH dynamics can be modified, if supernovae are powerful enough to disrupt gas clumps forming in the discs. A general introduction to the work is reported in Chapter 1. In Chapter 3 I discuss the first study about an alternative model for seed BH formation. In Chapter 4, instead, I describe the second study concerning the evolution of the MBH pair in the intermediate stages of a galaxy merger. The reader interested in the main results of the work can directly move to Chapters 3 and 4. Finally, Chapter 5 reports my conclusions

Hydrodynamical simulations of the tidal stripping of binary stars by massive black holes

In a galactic nucleus, a star on a low angular momentum orbit around the central massive black hole can be fully or partially disrupted by the black hole tidal field, lighting up the compact object via gas accretion. This phenomenon can repeat if the star, not fully disrupted, is on a closed orbit. Because of the multiplicity of stars in binary systems, also binary stars may experience in pairs such a fate, immediately after being tidally separated. The consumption of both the binary components by the black hole is expected to power a double-peaked flare. In this paper, we perform for the first time, with GADGET2, a suite of smoothed particle hydrodynamics simulations of binary stars around a galactic central black hole in the Newtonian regime. We show that accretion luminosity light curves from double tidal disruptions reveal a more prominent knee, rather than a double peak, when decreasing the impact parameter of the encounter and when elevating the difference between the mass of the star which leaves the system after binary separation and the mass of the companion. The detection of a knee can anticipate the onset of periodic accretion luminosity flares if one of the stars, only partially disrupted, remains bound to the black hole after binary separation. Thus knees could be precursors of periodic flares, which can then be predicted, followed up and better modelled. Analytical estimates in the black hole mass range $10^5-10^8 \rm M_{\rm \odot}$ show that the knee signature is enhanced in the case of black holes of mass $10^6-10^7 \rm M_{\rm \odot}$.Comment: 14 pages, 6 figures, 8 tables. Accepted by MNRA

Massive black hole and gas dynamics in mergers of galaxy nuclei - II. Black hole sinking in star-forming nuclear discs

Mergers of gas-rich galaxies are key events in the hierarchical built-up of cosmic structures, and can lead to the formation of massive black hole binaries. By means of high-resolution hydrodynamical simulations we consider the late stages of a gas-rich major merger, detailing the dynamics of two circumnuclear discs, and of the hosted massive black holes during their pairing phase. During the merger gas clumps with masses of a fraction of the black hole mass form because of fragmentation. Such high-density gas is very effective in forming stars, and the most massive clumps can substantially perturb the black hole orbits. After $\sim 10$ Myr from the start of the merger a gravitationally bound black hole binary forms at a separation of a few parsecs, and soon after, the separation falls below our resolution limit of $0.39$ pc. At the time of binary formation the original discs are almost completely disrupted because of SNa feedback, while on pc scales the residual gas settles in a circumbinary disc with mass $\sim 10^5 M_\odot$. We also test that binary dynamics is robust against the details of the SNa feedback employed in the simulations, while gas dynamics is not. We finally highlight the importance of the SNa time-scale on our results.Comment: 10 pages, 11 figures, MNRAS in pres

Black holes in galactic nuclei: seed formation from stellar mass black holes and massive black hole pairing in galaxy mergers.

Black holes (BHs) are a very important class of astrophysical objects. They are the most compact objects in the Universe, hence they represent the most extreme sources of gravity. BHs come in two flavours: the stellar mass BHs (SBHs) relic of young massive stars (1−20M⊙) and the massive BHs (MBHs), with masses of 106−109M⊙, dwelling in the nuclei of the most massive galaxies. While the formation mechanisms of SBHs are well understood, no clear consensus exists about MBH formation. According to the Soltan arguments (Soltan, 1982), MBHs gain the largest fraction of their mass via radiative efficient accretion of gas. As a consequence, we expect that MBH formed early in the Universe as smaller mass seeds. Recently, observations of high redshift quasars (e.g.; Mortlock et al., 2011; Fan et al., 2006) showed that MBHs with masses above 109M⊙ were already in place when the Universe was less than 1 Gyr old and posed tight constraints on the models for the formation and growth of MBHs. Two main scenarios have been developed for MBH seed formation: the light seed scenario, where seeds formed as relic of the first generation of stars with masses of up to few hundred solar masses (Madau & Rees, 2001), and the heavy seed scenarios, where seeds formed from the direct collapse of massive gas clouds in primordial haloes with masses of up to few 105M⊙ (Haehnelt & Rees, 1993). Despite the large number of studies about MBH formation models, each model still has its own caveats, which make the study of MBH formation worth of further investigations. According to the -CDM cosmology, galaxies form when gas cools down within dark matter haloes, which assembly in a hierarchical fashion from small density perturbations. Galaxies grow via accretion and mergers, and the central MBHs evolve in the same way. So, when a galaxy merger occur, the MBHs hosted in the nucleus of the galaxy progenitors can sink towards the centre of the merger remnant, forming a MBH binary (MBHB). Despite galaxy mergers are usually observed, no clear detections of MBHBs exist to date. The formation and evolution of MBHBs is a complex process, since it occurs in a rapidly varying environment where gas, star formation and SNa feedback play a pivotal role. Several studies have been performed to date, but a clear understanding of the whole process is still far from being reached. In this thesis I cover both aspects of MBH formation and evolution. In the first study I consider an alternative route for seed BH formation. Using two different codes, the AMR code RAMSES (Teyssier, 2002) and the meshfree code GIZMO (Hopkins, 2015), I studied the evolution of a single massive circum-nuclear gaseous disc embedding a population of SBHs. The disc was subject to radiative cooling, star formation and supernova feedback and becomes unstable to fragmentation, which led to the formation of clumps as massive as 104 − 105M⊙. My simulations showed that during the disc evolution, some SBHs can be gravitationally captured by a clump. Within the clumps, such BHs can experience episodes of super-critical accretion, which make them grow up to 103 − 104M⊙ in few Myr. Thanks to the very low radiative efficiency associated to the slim accretion disc (Abramowicz et al., 1988), the energy released to the surrounding gas is too small to halt the accretion flow, hence BHs can accrete almost unimpeded until one of these events occur: the clump is totally accreted by the BH, the clump is consumed by star formation or the clump is destroyed by supernova explosions. In the second study, instead, I consider the intermediate stages of a galaxy merger, when the MBHs originally dwelling in the centre of their own progenitor galaxies reach few hundred separations in the nucleus of the merger remnant. I assumed that each MBH was embedded in a self-gravitating circumnuclear gaseous disc. With the code RAMSES I studied the evolution of the MBHs and their surrounding discs, including physical processes like radiative cooling, star formation and supernova feedback, which are implemented in the code as sub-grid recipes. First, I implemented a new refinement prescription aimed at improving the orbital evolution of massive particles, an already known major issue in AMR codes, like observed by Gabor & Bournaud (2013); Dubois et al. (2014). Secondly, I evolved the discs assuming different sub-grid recipes to study how the MBH and gas dynamics could be affected by the different choices. I found that the MBH dynamics is almost independent of the physical modelling, if one assumes that no previous star formation occurred in the discs, while the gas evolution and its final distribution can be significantly affected. On the other side, if one assumes that star formation was already ongoing, even the BH dynamics can be modified, if supernovae are powerful enough to disrupt gas clumps forming in the discs. A general introduction to the work is reported in Chapter 1. In Chapter 3 I discuss the first study about an alternative model for seed BH formation. In Chapter 4, instead, I describe the second study concerning the evolution of the MBH pair in the intermediate stages of a galaxy merger. The reader interested in the main results of the work can directly move to Chapters 3 and 4. Finally, Chapter 5 reports my conclusions

Constraining the high redshift formation of black hole seeds in nuclear star clusters with gas inflows

In this paper we explore a possible route of black hole seed formation that appeal to a model by Davies, Miller & Bellovary who considered the case of the dynamical collapse of a dense cluster of stellar black holes subjected to an inflow of gas. Here, we explore this case in a broad cosmological context. The working hypotheses are that (i) nuclear star clusters form at high redshifts in pre-galactic discs hosted in dark matter halos, providing a suitable environment for the formation of stellar black holes in their cores, (ii) major central inflows of gas occur onto these clusters due to instabilities seeded in the growing discs and/or to mergers with other gas-rich halos, and that (iii) following the inflow, stellar black holes in the core avoid ejection due to the steepening to the potential well, leading to core collapse and the formation of a massive seed of $<~ 1000\, \rm M_\odot$. We simulate a cosmological box tracing the build up of the dark matter halos and there embedded baryons, and explore cluster evolution with a semi-analytical model. We show that this route is feasible, peaks at redshifts $z <~ 10$ and occurs in concomitance with the formation of seeds from other channels. The channel is competitive relative to others, and is independent of the metal content of the parent cluster. This mechanism of gas driven core collapse requires inflows with masses at least ten times larger than the mass of the parent star cluster, occurring on timescales shorter than the evaporation/ejection time of the stellar black holes from the core. In this respect, the results provide upper limit to the frequency of this process

Globular Cluster Formation from Colliding Substructure

We investigate a scenario where the formation of Globular Clusters (GCs) is triggered by high-speed collisions between infalling atomic-cooling subhalos during the assembly of the main galaxy host, a special dynamical mode of star formation that operates at high gas pressures and is intimately tied to LCDM hierarchical galaxy assembly. The proposed mechanism would give origin to "naked" globulars, as colliding dark matter subhalos and their stars will simply pass through one another while the warm gas within them clashes at highly supersonic speed and decouples from the collisionless component, in a process reminiscent of the Bullet galaxy cluster. We find that the resulting shock-compressed layer cools on a timescale that is typically shorter than the crossing time, first by atomic line emission and then via fine-structure metal-line emission, and is subject to gravitational instability and fragmentation. Through a combination of kinetic theory approximation and high-resolution $N$-body simulations, we show that this model may produce: (a) a GC number-halo mass relation that is linear down to dwarf galaxy scales and agrees with the trend observed over five orders of magnitude in galaxy mass; (b) a population of old globulars with a median age of 12 Gyr and an age spread similar to that observed; (c) a spatial distribution that is biased relative to the overall mass profile of the host; and (d) a bimodal metallicity distribution with a spread similar to that observed in massive galaxies.Comment: 15 pages, 5 figures, accepted for publication by the Astrophysical Journa

Difficulties in Mid-Infrared selection of AGN in dwarf galaxies

While massive black holes (MBHs) are known to inhabit all massive galaxies, their ubiquitous presence in dwarf galaxies has not been confirmed yet, with only a limited number of sources detected so far. Recently, some studies proposed infrared emission as an alternative way to identify MBHs in dwarfs, based on a similar approach usually applied to quasars. In this study, by accurately combining optical and infrared data taking into account resolution effects and source overlapping, we investigate in detail the possible limitations of this approach with current ground-based facilities, finding a quite low ($\sim$0.4 per cent) fraction of active MBH in dwarfs that are luminous in mid-infrared, consistent with several previous results. Our results suggest that the infrared selection is strongly affected by several limitations that make the identification of MBHs in dwarf galaxies currently prohibitive, especially because of the very poor resolution compared to optical surveys, and the likely contamination by nearby sources, although we find a few good candidates worth further follow-ups. Optical, X-ray and radio observations, therefore, still represent the most secure way to search for MBH in dwarfs.Comment: 7 pages, 7 figures, 1 table, accepted for publication on MNRA